1. Field of the Invention
The present invention relates to a manufacturing method of a magnetoresistive element and a vacuum processing apparatus using the manufacturing method.
2. Description of the Related Art
With the increase in the amount of information in recent years, electronic devices are desired to consume less energy, and memories are desired to operate at a high speed and desired to be nonvolatile. Currently used memories include a DRAM (Dynamic Random Access Memory, hereinafter referred to as a DRAM), a flash memory, and the like that use accumulation of electrical charges. The DRAM is used as a main memory of a computer, and is a volatile memory that loses memory when the power supply is cut off.
It is necessary to perform rewriting with a regular interval of time in order to hold data during operation, and this increases the consumed electric power. On the other hand, the flash memory is a non-volatile memory, but the writing time of information is slow, i.e., in the order of microseconds. It has been expected to apply a magnetoresistive memory (Magnetic Random Access Memory: MRAM) as a non-volatile memory consuming low electric power and operating at a high speed without having such disadvantages explained above.
FIGS. 1A and 1B illustrate a basic structure of a magnetoresistive memory. The magnetoresistive memory includes a magnetoresistive element 103 installed between a bit line 101 and a word line 102, and a transistor 104 for selecting each resistance element. The magnetoresistive element 103 has such a structure in which a magnesium oxide (MgO) 107 is sandwiched between a free layer 105, i.e., a magnetic film in which a direction of magnetization can be reversed by an external magnetic field or spin injection, and a fixed layer 106 which is a magnetic film in which a direction of magnetization is kept fixed.
The magnetoresistive element 103 is such that, as illustrated in FIG. 1A, when the directions of the magnetizations (arrow in the drawing) of the free layer 105 and the fixed layer 106 are in parallel to each other, the resistance of the magnetoresistive element 103 becomes low, and as illustrated in FIG. 1B, when the directions of the magnetizations of the free layer 105 and the fixed layer 106 are in antiparallel to each other, the resistance of the magnetoresistive element 103 becomes high. A memory in which the resistance difference of this magnetoresistive element 103 is associated with I/O signals is a magnetoresistive memory, and the manufacturing of the magnetoresistive element, which is the core of the magnetoresistive memory, is important.
In FIGS. 1A and 1B, the directions of the magnetizations of the free layer 105 and the fixed layer 106 are in a vertical direction with respect to the film surface, but they may also be in a horizontal direction with respect to the film surface. In FIGS. 1A and 1B, the free layer 105 is formed at the upper side of the magnesium oxide (MgO), and the fixed layer 106 is formed at the lower side of the magnesium oxide (MgO), but the positions of the free layer 105 and the fixed layer 106 may be reversed. More specifically, the fixed layer 106 may be formed at the upper side of the magnesium oxide (MgO), and the free layer 105 may be formed at the lower side of the magnesium oxide (MgO). Not only the magnetoresistive memory but also a magnetic head, a magnetic sensor and the like using this magnetoresistive element have been developed rapidly.
The manufacturing of the magnetoresistive element requires a technique of micromachining with dry etching for processing a magnetic film including chemical elements such as Fe, Co, Ni, and the like used in the free layer and the fixed layer and a barrier layer made of magnesium oxide (MgO). Two types of methods as illustrated in FIGS. 11A and 11B have been considered as a processing method of a magnetoresistive element using plasma etching. The first method is, as illustrated in step 1 in FIG. 11A, a method for collectively processing a free layer 1102, a magnesium oxide (MgO) 1103, and a fixed layer 1104 formed on an Si wafer 1101 by using a mask 1105.
The second method is a method in which, as illustrated in FIG. 11B, etching is interrupted after a free layer 1102 is processed by using a mask 1105 (step 1), and after the protection film 1106 is thereafter formed (step 2), the magnesium oxide (MgO) 1103 and the fixed layer 1104 are processed (step 3). In FIGS. 11A and 11B, the free layer 1102 is formed at the upper side of the magnesium oxide (MgO) 1103, and the fixed layer 1104 is formed at the lower side of the magnesium oxide (MgO) 1103, but the positions of the free layer 1102 and the fixed layer 1104 may be reversed.
In step 3 in FIG. 11B, the magnesium oxide (MgO) 1103 and the fixed layer 1104 are processed by using the protection film 1106, but in a case where the protection film cannot obtain a sufficient level of etching resistivity against the magnetic film, a mask may be formed on the protection film 1106 before step 3 is performed, and the magnesium oxide (MgO) 1103 and the fixed layer 1104 may be processed by using the mask. In the method as illustrated in FIGS. 11A and 11B, in step 1 of FIG. 11A and step 1 and step 3 of FIG. 11B, it is necessary to have micromachining technique for processing the magnetic film and the magnesium oxide (MgO) by using plasma etching, and two methods including a method using ion beam etching and a method using plasma etching have been considered as this method.
In the ion beam etching, a processing gas is introduced to a depressurized ion source, the gas is into plasma by applying a radio frequency electric power to a processing chamber via a flat-plate antenna, a coil-shaped antenna, and the like, and the ions generated therefrom are accelerated and drawn from the ion source into the processing chamber with the voltage applied to several grid electrodes, and the ion beam etching advances as the drawn ions are emitted onto the substrate disposed in the processing chamber.
As the ion source, there are various methods such as magneto-active field microwave type, Inductively Coupled Plasma (Inductively Coupled Plasma: ICP) type, and the like, which are different in the method for generating plasma. In order to make it less likely for the wafer to be charged to a positive polarity with the emitted ions, a neutralizing gun for emitting electrons may be installed in a processing chamber. In this method, with the radio frequency electric power, the amount of emission of ion can be controlled so that the energy of the ion is controlled in accordance with a voltage applied to the grid electrodes.
The wafer stage in the processing chamber has a rotation mechanism and an inclination mechanism, and the uniformity of the beam in the circumferential direction can be improved by rotating the wafer at a constant speed during processing, and the angle of the ion emitted onto the wafer can be controlled with the inclination mechanism. In this method, rare gases such He, Ne, Ar, Kr, Xe are generally used as the gas introduced in to the ion source and made into plasma, but it may be possible to mix reactive gases such as hydrogen, nitrogen, oxygen, and the like.
On the other hand, in the plasma etching, the processing gas is introduced into the depressurized processing chamber, and the gas is made into plasma when the source power supply provides the radio frequency electric power (hereinafter referred to as a source electric power) into the processing chamber via the flat-plate antenna, the coil-shaped antenna, and the like, so that the plasma etching advances when ions and radicals generated therefrom are directly emitted onto the substrate. As the plasma source, there are various methods such as magneto-active field microwave type, Inductively Coupled Plasma (ICP) type, Capacitively Coupled Plasma (CCP) type, and the like which are different in the method for generating plasma.
In order to actively draw the ions in the plasma into the wafer, a radio frequency electric power (which may be hereinafter referred to as a bias electric power) may also be applied to a stage on which the wafer is installed. As the method of processing the magnetoresistive element using the plasma etching, etching methods using plasma including oxygen atoms and hydrogen atoms have been considered, e.g., a method for making a mixed gas of CO and NH3 into plasma disclosed in Japanese Patent Laid-Open No. H08-253881 and a method for making CH3OH gas into plasma disclosed in Japanese Patent Laid-Open No. 2005-042143 have been considered.
According to the magnetic film processing method based on the ion beam etching and the plasma etching explained above, in a case where an emitted ion collides with the magnesium oxide (MgO), oxygen of which mass is lighter than magnesium is selectively removed from magnesium oxide (MgO), and magnesium oxide (MgO) is caused to be in a low oxidation state (reduced), so that the electrical characteristics of the magnetoresistive element are deteriorated. There is a problem in that, in the plasma etching, reduction of magnesium oxide (MgO) which is the barrier layer is promoted by the hydrogen ions and the hydrogen radicals generated in the plasma, and the electrical characteristics of the magnetoresistive element are deteriorated.
On the other hand, in a case where plasma generated using a gas including oxygen chemical elements is used in ion beam etching and plasma etching, the magnetic film used in the fixed layer and the free layer is oxidized by the oxygen radicals and the oxygen ions used in the plasma, so that the electrical characteristics of the magnetoresistive element are deteriorated. Therefore, it is necessary to achieve a process that can realize both of suppressing reduction of magnesium oxide (MgO) and suppressing oxidization of the magnetic film.
As the method for recovering a damage caused by the magnesium oxide (MgO), “Novel oxygen showering process (OSP) for extreme damage suppression of sub-20 nm high density p-MTJ array without IBE treatment” (J. H. Jeong and T. Endoh, Symposium on VLSI Technology Digest of Technical Papers (2015)) reports a recovery method for etching a magnetic film by using plasma including hydrogen chemical elements and thereafter emitting ozone gas to the magnetoresistive element. As the method for recovering the damage caused by oxidation of the magnetic film, “Damage recovery by reductive chemistry after methanol-based plasma etch to fabricate magnetic tunnel junctions damage” (K. Kinoshita et al., Japanese Journal of Applied Physics 51 (2012)) reports a method for processing the magnetic film with an oxygen gas and a mixed gas of Ar and methanol and thereafter emitting plasma generated by He/H2 gas onto a magnetoresistive element, thus recovering a damage of the magnetic film.
Japanese Patent Laid-Open No. 2009-302550 discloses, in order to etch a magnetic film with plasma including hydrogen atoms and oxygen atoms and thereafter remove a damaged layer formed during etching of a magnetic film, a method for performing reduction processing in a radical processing chamber using plasma generated with reducing gas such as hydrogen and ammonia and thereafter forming a protection film and a method for performing etching of the magnetic film, reduction processing, and formation of the protection film in a multi-chamber process in vacuum.
As the reduction of oxidized metal materials, Japanese Patent Laid-Open No. 2009-206472 discloses a method for oxidizing a metal material such as Cu by using plasma of oxygen gas after predetermined etching, then putting a wafer in atmosphere to clean the wafer with wet cleaning, thereafter performing reduction processing using a formic acid gas and the like in order to remove the plasma of the oxygen gas and metal oxides formed during conveyance in the atmosphere, and thereafter forming a barrier film according to a chemical vapor deposition (CVD) method using vapors of organic metal compounds, and a method for performing reduction processing using the formic acid gas and the like and a CVD method using vapors of organic metal compounds which are performed in a multi-chamber process in vacuum.